ldns-keygen: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/ldns-keygen - from package: ldns

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ldns-keygen - from package: ldnsutils

You may install the required package with command apt-get install {package}

Error in other languages:
ldns-keygen: bevel nie gevind nie
ldns-keygen: comandă negăsită
ldns-keygen: comando não encontrado
ldns-keygen: commande introuvable
ldns-keygen: command not found
ldns-keygen: käsku ei ole
ldns-keygen: không tìm thấy lệnh
ldns-keygen: komanda nerasta
ldns-keygen: Kommando nicht gefunden.
ldns-keygen: kommandot finns inte
ldns-keygen: komut yok
ldns-keygen: nie znaleziono polecenia
ldns-keygen: níor aimsíodh an t-ordú
ldns-keygen: no se encontró la orden
ldns-keygen: no s'ha trobat l'ordre
ldns-keygen: opdracht niet gevonden
ldns-keygen: parancs nem található
ldns-keygen: perintah tidak ditemukan
ldns-keygen: príkaz nenájdený
ldns-keygen: příkaz nenalezen
ldns-keygen: команда не найдена
ldns-keygen: командата не е открита
ldns-keygen: 命令找不到
ldns-keygen: コマンドが見つかりません